In the last 20 to 30 years, there has been a veritable explosion of high fidelity equipment intended for home use. During this period of time, available equipment became more and more sophisticated; for example, stereophonic equipment has almost wholly displaced monophonic equipment. Other advances have been made both in the signal chain as well as in associated devices for control and/or protection purposes.
As those skilled in the art are aware, the primary object of such equipment is to increase the signal level to the extent necessary to drive whatever transducers are included in the system, while at the same time, maintaining as linear a characteristic as is possible. Since signal source levels vary, and since desirable audio listening levels can also vary, the equipment includes, almost universally, volume controls to vary the power output. For many years it has been known that the amplifiers included in the equipment were subject to clipping as a result of setting the amplifier for a relatively high power output, providing the amplifier with a relatively high input signal, or a combination of these factors. Clipping, of course, introduces distortion and, to that extent, thus destroys the desired linearity.
Ozawa et al in U.S. Pat. No. 3,761,775, illustrate a protective circuit which includes a device to compare the amplifier input with a modified form of the amplifier output which is adapted to open circuit the amplifier output if substantial differences exist. Ozawa et al teach that this arrangement can be employed to remedy clipping. As a device for minimizing clipping, this arrangement has at least two disadvantages. In the first place, an attenuation circuit is required to attenuate the output signal by approximately the same factor as the amplification factor of the amplifier. Since most of the amplifiers that are in use today in high fidelity equipment are feedback amplifiers, the attenuation circuit merely duplicates circuitry already included in the amplifier. Although the attenuation circuit, as illustrated in Ozawa is merely a voltage divider, in actual practice, phase correction would also be necessary requiring the addition of at least a capacitor. A second more significant disadvantage to the Ozawa arrangement is that in the presence of clipping, the output signal is removed from the loudspeaker. This appears to be an example of "cutting off your nose to spite your face". Particularly, the presence of clipping would distort the output signal and could disturb a listener. Removing the output signal entirely does not appear to be a desirable solution. Instead, what is desired is an arrangement to minimize the effect of clipping while retaining the desired output signal.
Suzuki et al, in U.S. Pat. No. 3,891,933, teach an arrangement which eliminates the first disadvantage mentioned above with regard to Ozawa, but their arrangement is no better than Ozawa with regard to the second disadvantage above-mentioned.